This invention relates to electrical power control and, more particularly, to a method and apparatus for remotely controlling an electrical appliance connected across an electrical power line.
It is often desirable to remotely control an appliance connected to an electrical outlet. If the plan for such remote control is known before the electrical wiring is installed, wall switches can be provided at the appropriate locations to control application of electrical power to the outlets to which the appliances are to be connected. If the user of the building wishes to establish a plan for remote control of appliances after the electrical wiring is in place, there are known techniques for signalling a switch at the appliance over the existing electrical wiring to turn the appliance on and off.
One such technique described in Durkee U.S. Pat. No. 3,689,886 transmits one of a plurality of high frequency tone signals over power lines to control an electrical appliance; each frequency represents a separate control channel. A filter is provided for each remote switch to select the frequency corresponding to the particular channel. The filtered tone signal received at a remote switch is rectified and integrated to produce a switch actuating signal. Durkee uses time division multiplexing to increase the effective number of channels. The total number of channels is the product of the number of different frequency tone signals and the number of time slot divisions. In addition to the likelihood that high frequency tone signals will interfere with communication equipment such as radio and television connected to the power line, Durkee's technique has the shortcomings normally associated with high frequency equipment such as high cost, instability, and lack of reliability.
Another technique disclosed in Woods U.S. Pat. No. 3,594,584 uses for signalling periodic pulses at a frequency several times higher than the line frequency and in no particular phase relationship to the alternating current. At the remote outlets, the pulses are rectified and integrated to produce a switch actuating signal. Woods only makes provision for one control channel, which severely limits the usefulness of his technique, and his switch is susceptible to accidental actuation by spurious signals and noise on the power line.